Those are exactly the guidelines I read while researching vaccinations earlier this week. I read that some vets believe every 3 years is adequate (particularly in younger animals) but that older dogs have usually built up enough immunity to last them a lifetime.

Thankfully, Rusty doesn't have any major health problems for a dog of her age. She does have arthritis in her hips and knees (though she's doing quite well on daily glucosamine and aspirin) and she was diagnosed with and surgically treated for an autoimmune eye condition (KCS) during her first year of life. The eye condition does make me worry that she's immunocompromised to some extent.